In general, an image reading device includes an image scanning portion configured to scan a document sheet placed on a transparent document sheet table while detecting an amount of light emitted from the document sheet. The image scanning portion includes a light emitting portion configured to emit light toward the document sheet; and an image sensor configured to receive light reflected from a linear area of the document sheet that extends along a main scanning direction, and the image scanning portion scans the document sheet along a sub scanning direction. The image sensor outputs a data sequence representing amounts of light detected with respect to pixels of the linear area of the document sheet. It is noted that the sub scanning direction is perpendicular to the main scanning direction.
Image data for a secondary image of the document sheet is obtained by combining, along the sub scanning direction, data sequences representing image densities of the linear areas obtained through the image scanning portion. In addition, an image processing apparatus such as a copier includes an image reading portion and an image forming portion. The image forming portion of the copier forms the secondary image on a recording sheet by forming, sequentially on the recording sheet, linear images based on the data sequences representing image densities of the linear areas obtained through the image scanning portion.
In addition, there is known a technique where normal image density data and comparison image density data are compared, and an area including pixels that differ greatly in density is identified as a document sheet area, wherein the normal image density data is obtained when the light emitting portion is set to emit a normal amount of light, and the comparison image density data is obtained when the light emitting portion is set to emit a different amount of light (see, for example, PTL-1).